Cooling Rate Effects on Local Atomic Structure and Soft Magnetic Properties of the Fe74B20Nb2Hf2Si2 Amorphous Alloy

Abstract

In  this  study, we  investigate  the  influence  of  cooling  rate  on  the local  atomic  structure  and  soft  magnetic  properties  of  the Fe₇₄B₂₀Nb₂Hf₂Si₂  amorphous  alloy. Two bulk amorphous samples with  dimensions  of  70.0 x 1.0 x 1.0 mm³  (lower  cooling  rate)  and  50.0 x 30.0 x 0.5 mm³ (higher cooling rate) were successfully fabricated by suction casting into a water-cooled copper mold. The fully amorphous nature of both samples was confirmed by Mössbauer spectroscopy; a characteristic broad and asymmetric sextet — typical of soft magnetic amorphous alloys — was recorded, indicating that no crystallization occurred despite the different cooling conditions. However, variations in hyperfine parameters revealed subtle changes in the local atomic environment induced by the cooling rate. The quadrupole splitting (Δ) increased from 0.046~mm/s in the faster-cooled sample to 0.051 mm/s in the slower-cooled sample, suggesting enhanced structural relaxation and a more equilibrated short-range atomic arrangement. In contrast, the average hyperfine magnetic field ((B_hf)_avg) slightly decreased from 20.1 to 19.9 T, indicating minor modifications in Fe–Fe exchange interactions and a more homogeneous amorphous structure. Magnetic measurements also revealed that the slower-cooled sample exhibited superior soft magnetic performance, with saturation magnetization (M_s) increasing from 1.08 to 1.14 T and coercivity (H_c) decreasing from 2.3 to 1.5 A/m. The improved magnetic properties are attributed to structural relaxation, reduced residual internal stresses, and a decrease in domain wall pinning sites and magnetoelastic anisotropy. These findings demonstrate that controlled slower cooling in Fe-based amorphous alloys promotes beneficial atomic-scale relaxation and significantly enhances soft magnetic properties, making this alloy a promising candidate for advanced soft magnetic applications.